1. Field of the Invention
The present invention relates to a sub-access point (AP), system, and method for adjusting the power of a transmission signal, and more particularly, to a sub-AP system and method for adjusting the power of a transmission signal, in which the power of a transmission signal output from the sub-AP is adjusted based on received signal strength indicator (RSSI) readings and a frame error rate (FER) of a main AP, thereby eliminating shadow areas and minimizing the interference between the main AP and the sub-AP.
2. Description of the Related Art
In general, in a wireless local area network (LAN), a home or a predetermined region can be serviced by using a single access point (AP).
In some cases, however, the home or the predetermined region cannot be serviced by using a single AP because of the building structure or topography—a problem which will be described in detail below with reference to FIG. 1.
FIG. 1 is a diagram showing a received signal strength indicator (RSSI) distribution in a case where a conventional AP is installed on a wall of a living room.
Referring to FIG. 1, a wireless LAN AP is generally installed in an area to facilitate efficient wireless communication, e.g., in a living room of an apartment. The beam pattern of an antenna is generally unidirectional, and thus, stable communication can be maintained in the living room.
However, a room adjacent to the living room and shielded by walls may get only a weak signal, if any, transmitted by the AP installed in the living room. Thus, in this adjacent room the data transfer rate may be considerably lower, or communication may be unsuccessful. A room such as this is called a shadow area, which is an area where radio waves can hardly be received because of the building structure or topography.
In other words, as illustrated in FIG. 1, the closer to the AP, the higher the RSSI. Portions of the living room with higher RSSIs are represented by darker shading. However, since in a room other than the living room it is difficult to receive a signal transmitted by the AP, portions of the room are rendered in lighter shading to represent the low RSSI.
In short, shadow areas may be generated in rooms other than the living room. Accordingly, a Voice over Internet Protocol (VoIP) service cannot be provided to such a shadow area.
In order to help solve this problem with shadow areas, and other problems, an additional AP with a lower power than the AP of the living room may be installed in one of the shadow areas, thereby expanding the range of wireless communication for a user.
In this case, however, if the two APs use the same channel, they may interfere with each other, thereby deteriorating the performance of the wireless network. Further, the additional AP is designed to transmit a signal with a fixed power, which may be sometimes higher than what is needed.
In order to reduce interference between the two APs, two different channels may be allocated to the APs. In this situation, however, the entire network capacity decreases especially in a densely populated place, such as an apartment building, because of a shortage of available channels.
Moreover, the additional AP transmits a signal with a fixed power regardless of the location of the AP in the living room. Thus, if a user relocates the AP in the living room, new shadow areas may be generated.
Therefore, it is necessary to develop a system and method which, in the illustrative and non-limiting scenario discussed above, can adjust the transmission power of the additional AP and can minimize interference between the AP installed in the living room and the additional AP, even when the two APs use the same channel.
Korean Patent Laid-Open Publication No. 1999-031807 entitled “Method and Apparatus for Controlling the Transmission Power of Wireless LAN (WLAN) station” discloses a method of controlling the transmission power of a WLAN station which includes: allowing a client station to issue a request to an AP for controlling the transmission power if it receives a low power signal; and allowing the AP to issue a request for switching of transmission power modes to all the stations in a wireless LAN. In this patented method, however, even when the transmission power levels of the stations in the wireless LAN are successfully controlled, it is still very difficult to completely eliminate shadow areas simply by using a single AP.